Three refineries send their gasoline product to two terminals. The capacities of the refineries are estimated at 200,000, 250,000 and 300,000 bb per day. The demands at the terminals are known to be 400,000 and 450,000 bb per day. Any demand that cannot be satisfied from the refineries is acquired from other sources. The gasoline product is transported to the terminals via a net work of terminals that are boosted by three pumping stations. The following figure summarizes the links of the network together with the capacity of each pipeline.

Nodes: 1,2&3 are refineries

4,5&6 : pumping stations

7&8 are terminals
 
a) Model the network problem to determine how much flow should be passing through each pumping station?  

b) Solve the problem using Excel solver.